Implement BlockDeviceMappings for AWS::EC2::Instance

Now in Heat, the AWS::EC2::Instance resource only has 'Volumes' property to
indicate the volumes to be attached, but there are two ways defining volumes
in AWSCloudFormation, 'Volumes' and 'BlockDeviceMappings', see:

http://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-properties-ec2-instance.html

1. 'Volumes' support the 'volume_id', user can specify the volume to be attached
   to the instance. This way has been implemented in Heat, but it's not a good way
   for batch creation because one volume can't be attached to many instances.

2. 'BlockDeviceMappings' support the 'snapshot_id', user can specify a snapshot,
   then a volume will be created from the snapshot, and the volume will be attached
   to the instance. This way is a good way for batch creation.

Nova supports to create a server with a block device mapping:

http://docs.openstack.org/api/openstack-compute/2/content/ext-os-block-device-mapping-v2-boot.html

So, we should support the 'BlockDeviceMappings' for AWS::EC2::Instance resource.